1. Field of the Invention
The present invention relates to a microwave baking furnace that bakes an object to be baked which is made of pottery materials, fine ceramics materials, etc. to produce a baked object.
2. Description of the Related Art
Recently, a technique that bakes pottery materials or fine ceramics by microwave heating has been suggested, and has already been put to practical use.
There are various types of microwave baking furnaces for microwave baking of ceramics, for example, a type in which an object to be baked such as ceramics self-heats by microwaves to bake itself and a type in which a heating material that generates heat by microwaves is disposed near an object to be baked and the object to be baked is baked by heat generated by the heating material. Japanese Patent Laid-Open No. 6-345541 (Pages 2 and 3, FIG. 1 discloses the former type of baking furnace.
As the latter type of microwave baking furnace, a baking furnace whose inner peripheral walls are made of a heating material that self-heats by microwaves is suggested (see Japanese Patent Laid-Open No. 2-275777 (Page 3, FIG. 1)). This baking furnace includes a cylindrical container made of a microwave-transmittable heat insulating member are received within a microwave oven and a cylindrical element made of a baked silicon carbide material and disposed in the container. In this baking furnace, using the inside of the cylindrical element as a baking portion, an object to be baked is put into the baking portion, and microwaves are radiated to the baked silicon carbide material, which causes it to generate heat to bake the object to be baked.
As a baking furnace employing both types together, a baking furnace is suggested (see Japanese Patent Laid-Open No. 7-318262 (Page 3, FIG. 1)) which includes a heating container composed mainly of a substance having a high microwave loss; a heat insulating member covering the outside of the heating container and composed mainly of a substance having a low microwave loss; an opening formed in the heating container; and a microwave radiating device that radiates microwaves into the heating container via the heat insulating member and to an object to be baked in the heating container via the opening in the heating container. In this baking furnace, the temperature distribution in the thickness direction can be made flatter.
When a homogenous object is baked by microwave heating, theoretically, every part of the object to be baked is heated uniformly by microwaves. However, during baking process, since the atmospheric temperature of the microwave baking furnace is much lower than the surface temperature of the object to be baked, the object to be baked radiates heat from its surfaces. Consequently, a temperature gradient is caused between the central portion and the surfaces of the object to be baked, which is likely to induce cracks in the object to be baked.
Moreover, as a characteristic of microwave heating, if an object to be baked is made of the same material, a portion of the object having a higher temperature has a larger dielectric loss. Therefore, once a temperature gradient is caused, the microwave absorption rate of a portion having a higher temperature increases, and thus microwave absorption rates vary considerably from portion to portion, which causes local heating of the object. In this way, once a temperature gradient is caused, temperatures vary considerably from portion to portion by microwave heating, which promotes cracks in the object to be baked.
Further, when alumina, silica, etc., a main material of ceramics having a low dielectric loss at room temperature, is used as a raw material of an object to be baked during baking by microwave heating, there is a problem in that the energy effect of microwave heating in a low temperature range is low.
Thus, as a microwave baking furnace which can suppress the occurrence of a temperature gradient to reduce the occurrence of crack, as shown in FIG. 7, a microwave baking furnace having a heater 18 disposed therein to control the temperature of the microwave baking furnace is suggested (see Japanese Patent Laid-Open No. 6-345541)).
Moreover, a baking furnace, as shown in FIG. 8, including a baking chamber 26 partitioned to surround a whole object 20 to be baked with a blanket 19 that can self-heat by microwaves and microwave generating means 22 that radiate microwaves to the object to be baked disposed in the baking chamber 26, is suggested (see Japanese Patent Laid-Open No. 2002-130960 (Page 3, FIG. 1)). In such a baking furnace, heating value per unit volume by microwaves of the blanket is larger than that of the object to be baked, and the inner surface temperature of the blanket is substantially equal to that of the object to be baked.
This furnace is considered to be devised from an idea that, when an object is baked by microwaves, the object to be baked can be almost completely insulted from heat by completely surrounding the object to be baked with the blanket having microwave absorption characteristics equivalent to those of the object to be baked. In this case, the occurrence of heat gradient in the object can be prevented by radiation cooling, and the object can be baked more uniformly. However, when the object to be baked is baked while being surrounded by the blanket, microwave energy is absorbed by the blanket as well as the object to be baked. Therefore, there is a problem in that the amount of energy required for baking is considerably increased.
When the thickness of the blanket is made small to decrease the amount of energy consumed by the blanket, the blanket loses more thermal energy than that it obtains by microwaves. Therefore, a large temperature difference is caused between the inner surfaces of the blanket and the object to be baked. In order to solve the problem, it is sought not only to decrease the amount of energy required for baking the object to be baked but also to prevent the occurrence of a temperature gradient due to radiation cooling in the object to be baked.
In this furnace, the problem has been solved by making heating value per unit volume by microwave of the blanket larger than that of the object to be baked, and by making the temperature of the inner surfaces of the blanket equal to the surface temperature of the object to be baked.
In the microwave baking furnace additionally including a heater 18 that can implement a heat treatment independently, such as the microwave baking furnace disclosed in Japanese Patent Laid-Open No. 6-345541 (Pages 2 and 3, FIG. 1), since the heater 18 supplementarily heats the object in a low temperature range in which the energy effect of microwave heating is low, an object having a low dielectric loss at room temperature can be baked, and the energy efficiency for baking can be improved.
Further, as described in Japanese Patent Laid-Open No. 2002-130960 (Page 3, FIG. 1), a heat insulating property around the baking chamber can be improved, and the occurrence of a temperature gradient due to heat radiation can be prevented by covering the blanket which defines the baking chamber with another blanket having an excellent heat insulating property.
However, the microwave baking furnace described in each Patent Document has a complex structure, and thus requires high manufacturing cost. In addition, in the microwave baking furnace described in Japanese Patent Laid-Open No. 2002-130960 (Page 3, FIG. 1), although an effect of suppressing the occurrence of a temperature gradient can be obtained to some extent, the energy efficiency in a low temperature range is rarely improved.
In a microwave baking furnace having a metallic cavity radiated with microwaves and a microwave generating means, a baking chamber that receives an object to be baked provided in the cavity is surrounded by a heat insulating member having a low microwave absorption characteristic and a high heat insulating property. A microwave baking furnace shown in FIG. 6 can be considered as the microwave baking furnace which has the above structure and high efficiency.
The microwave baking furnace shown in FIG. 6 bakes pottery materials or fine ceramics by microwave heating, and includes a cavity that defines a microwave space 2; a magnetron as a microwave generating means that is connected to the cavity 3 via a waveguide 4, and that radiates microwaves to the cavity 3; a microwave stirring means 7 that stirs microwaves radiated to the cavity 3; and a blanket 19 that is disposed in the cavity 3 to surround an object 11 to be baked.
The cavity 3 is adapted to reflect microwaves to the microwave space 2 at least at the inner surfaces thereof, and to prevent microwave leakage.
The microwave stirring means 7 includes stirring blades 8 disposed in the cavity 3; a driving motor 9 disposed outside the cavity 3; and a rotation transmitting shaft 10 that transmits the rotation of the driving motor 9 to the stirring blades 8, and stirs the atmosphere in the cavity 3 with the rotation of the stirring blades 8.
The blanket 19 partitions the baking chamber 12 in which the object 11 to be baked is to be disposed, and has a double layer structure of a heat insulating member 15a and a substance 15b having a high microwave loss.
The heat insulating member 15a is made of a material that not only insulates heat but also transmits microwaves, specifically, alumina fiber, foamed alumina, etc.
As shown in FIG. 9, the heat insulating member 15a can suppress heat radiation to the outside from the baking chamber 12 or the blanket 19 better as the thickness thereof increases.
In FIG. 9, a curved line F1 represents a heat radiation characteristic in a case in which the thickness of a heat insulating member 15a is small, and a curved line F2 represents a heat radiation characteristic in a case in which the thickness of a heat insulating member 15a is increased as compared to that in the curved line F1. It is evident from the drawing that the heat insulating member 15a having an increased thickness can improve the heat insulating property better. In FIG. 9, the horizontal axis represents the temperature of the baking chamber 12, and the vertical axis represents the amount of heat radiated from the blanket 19 to the outside.
The substance 15b having a high microwave loss is made of a dielectric material that self-heats by microwaves radiated from the outside and transmits some of the radiated microwaves to the object 11 to be baked in the baking chamber 12.
In this case, it is preferable that the substance 15b having a high microwave loss be made of one of silicon carbide, silicon nitride, graphite and composites containing these as main components.
Meanwhile, when the microwave baking furnace 1 shown in FIG. 6 bakes ceramics that is the object 11 to be baked by microwaves, the substance (silicon carbide, etc.) 15b having a high microwave loss covers six surfaces or all surfaces of the baking chamber 12 uniformly.
Also, when the substance 15b having a high microwave loss covers six surfaces or all surfaces of the baking chamber 12 uniformly, there is a problem in that the substance 15b having a high microwave loss is heated locally by microwaves and the object 11 to be baked or the heat insulating member 15a is broken irrespective of the existence of the microwave stirring means 7.